A state of emergency was declared in some areas along the US east coast on Friday as experts warned Hurricane Sandy could contribute to a storm of "historic" proportions.

The hurricane left 41 people dead as it passed through the Caribbean and headed north. Sandy could strike the US coastline anywhere between Virginia and Massachusetts as early as Monday.

Meteorologists warned that weather conditions could be complicated as Sandy is expected to meet two separate weather systems somewhere over the north-east United States, resulting in high winds, heavy rain, extreme tides and perhaps even snow.

"It's looking like a very serious storm that could be historic," said Jeff Masters, meteorology director of the forecasting service Weather Underground. He compared the convergence of weather systems to the so-called "perfect storm" that struck off the coast of New England in 1991, although that storm hit a less populated area.

On Friday afternoon, the centre of the hurricane was moving slowly north around 430 miles south-southeast off the coast of South Carolina. Sandy was moving at 7mph with maximum sustained winds near 75mph. Forecasters said there was a 90% certainty that storm would make landfall on the east coast, althought it was too early to predict where it would come ashore.

Parts of Jamaica, Haiti and Cuba were left devastated as the hurricane swept over on Thursday and Friday, leaving at least 41 people dead. In Cuba, 11 people were killed in eastern Santiago and Guantanamo provinces, as authorities said Sandy was Cuba's deadliest storm since July 2005. One person died as Sandy passed through Jamaica and 16 were left dead in Haiti, where heavy rains from the storm's outer bands caused flooding in the impoverished and deforested country. A 66-year-old man died in the Bahamas after falling from his roof in upscale Lyford Cay late Thursday, as he attempted to repair a window shutter.

New York, Virginia and Maryland declared states of emergency on Friday. In Virginia, where Mitt Romney cancelled a rally planned for Sunday night, Governor Bob McDonnell told people to prepare ahead of the storm's arrival, warning that the state "could see severe weather lasting for 48 hours or more".

"In that scenario, saturated soil coupled with high winds could lead to major tree damage and extensive power outages," McDonnell said. "Virginians should make sure their family members, friends and neighbours are prepared for this extended weather event. I encourage all Virginians to gather batteries, blankets, water, canned goods, and other necessities prior to the anticipated onset of storm conditions late Saturday and early Sunday."

New York City mayor Mike Bloomberg said he would wait until Saturday before potentially issuing an evacuation order, but warned that the storm was moving at such a rate that "we're still not going to have a good sense of when and where it's going to hit land". The last major storm to threaten the northeast coast was Hurricane Irene which caused an estimated $15.8bn in damage in August last year, making it one of the costliest storms in history.

Bloomberg said that bridges may have to be closed in New York. The MTA suspends subway, bus and other transit services in advance of the arrival of sustained winds at 39mph or higher, he said, while power outages are also a possibility.

The National Oceanic and Atmospheric Administration said Friday that wherever the storm comes ashore, there will be 10in (254mm) of rain and extreme storm surges. Up to 2ft (0.6m) of snow should fall on West Virginia, with lighter snow in parts of Ohio and Pennsylvania.

"It's going to be a long-lasting event, two to three days of impact for a lot of people," said James Franklin, forecast chief for the National Hurricane Center.